(1) Field of the Invention
The present invention relates to a fixing device that thermally fixes an unfixed image formed on a recording sheet to the recording sheet, and an image forming apparatus provided with the fixing device.
(2) Related Art
In general, image forming apparatuses such as copiers, printers, facsimiles, and MFPs (Multiple Function Peripherals) have a structure that forms a toner image corresponding to image data, transfers the formed toner image onto a recording sheet, and fixes the transferred toner image to the recording sheet using a fixing device.
FIG. 7 is a cross-sectional schematic view for explaining a structure of a fixing device that forms a fixing nip using a pair of belts. This fixing device 70 has a pair of a first belt 71 and a second belt 72 that apply pressure, from left and right, to a recording sheet S which has had a toner image transferred thereon and is conveyed upward. FIG. 7 shows a state where the first belt 71 and the second belt 72 are separated from each other. However, in the actual fixing device 70, the first belt 71 and the second belt 72 are brought into pressing contact with each other at their respective upstream portions (lower side) and downstream portions (upper side) in the conveying direction of the recording sheet S, thereby forming a fixing nip between a portion of the first belt 71 and a portion of the second belt 72 that oppose each other.
The first belt 71 is suspended on a first upstream roller 73 disposed at the upstream side in the conveying direction of the recording sheet S and a first downstream roller 74 disposed at the downstream side of the conveying direction of the recording sheet S, with a predetermined tension (belt tension) applied thereto. The first belt 71 is rotated in a direction indicated by an arrow B, for example, according to the rotation of the first downstream roller 74 which is driven to rotate by a drive motor. Inside the first upstream roller 73, a heater lamp 77 formed in a linear fashion is arranged along an axis direction thereof.
The first upstream roller 73 and the first downstream roller 74 have the same radius r1 and are arranged in parallel with each other. The first upstream roller 73 is biased downward (toward upstream in the conveying direction of the recording sheet S) with a tension spring (not shown) so as to apply a predetermined belt tension F1 along the conveying direction of the recording sheet S, to the first belt 71.
The second belt 72 is suspended on a second upstream roller 75 disposed at the upstream side in the conveying direction of the recording sheet S (lower side) and a second downstream roller 76 disposed at the downstream side in the conveying direction of the recording sheet S (upper side), and rotates in a direction shown by an arrow C. The second upstream roller 75 and the second downstream roller 76 also have the same radius r1 as the first upstream roller 73 and the first downstream roller 74, and are arranged in parallel with each other. The second upstream roller 75 is also biased downward (toward upstream in the conveying direction of the recording sheet S) with a tension spring (not shown) so as to apply the belt tension F1 along the conveying direction of the recording sheet S, to the second belt 75.
The second upstream roller 75 and the second downstream roller 76 are pressed against the first upstream roller 73 and the first downstream roller 74, respectively, via the second belt 72 and the first belt 71, with an equal pressure P1. An upstream nip portion is formed between the first belt 71 and the second belt 72 pressed by the first upstream roller 73 and the second upstream roller 75. A downstream nip portion is formed between the first belt 71 and the second belt 72 pressed by the first downstream roller 74 and the second downstream roller 76. The upstream nip portion and the downstream nip portion have a substantially equal nip pressure. Between the upstream nip portion and the downstream nip portion is a free nip portion where a portion of the first belt 71 and a portion of the second belt 72 that oppose each other run in the same direction at the same speed.
In the fixing device 70 with the above-described structure, the recording sheet S having had the toner image transferred thereon enters the upstream nip portion, passes through the free nip portion and the downstream nip portion, and is discharged. While the recording sheet S passes through the upstream nip portion, the free nip portion, and the downstream nip portion, the unfixed toner image on the recording sheet S is heated by the heater lamp 77 into a molten state and fixed by being pressed to the recording sheet S.
FIG. 8 shows a graph showing distribution of a nip pressure at each of the upstream nip portion, the free nip portion, and the downstream nip portion of the fixing device with the above-described structure. At the free nip portion, while the first belt 71 and the second belt 72 are brought into contact with each other by the belt tension thereof, little nip pressure occurs.
Fixing of the toner image on the recording sheet is completed upon the toner image passing through the upstream nip portion, the free nip portion, and the downstream nip portion, and toner particles are in a molten state when passing through the free nip portion. Accordingly, if little nip pressure occurs at the free nip portion, image slippage may occur, that is, the toner image may slip out of the transfer position on the recording sheet before reaching the downstream nip portion due to vibration of the first belt 71, the second belt 72, or the recording sheet S. Additionally, in a case where the recording sheet S vibrates due to an insufficient pressure applied to the recording sheet S passing through the free nip portion, the recording sheet S may crease when entering the downstream nip portion.
Patent Document 1 (Japanese Patent Application Publication H11-174878) discloses a structure which is approximately the same as that of the fixing device 70 shown in FIG. 7. According to the disclosed structure, a heater is also provided inside the second downstream roller 76. This is in order to suppress occurrence of image defects when unfixed toner images transferred on both surfaces of the recording sheets are fixed.
Patent Document 2 (Japanese Patent Application Publication 2001-154516) also discloses a structure approximately the same as that of the fixing device 70 shown in FIG. 7. In the disclosed structure, the first downstream roller 74 is cooled to prevent creases of the recording sheet due to adhesive properties of toner.
However, neither the structure according to Patent Document 1 nor the structure according to Patent Document 2 is able to reliably prevent image slippage, creases of the recording sheet S, and the like. This is because according to these structures, the free nip portion is formed merely by the mutually opposing belt portions, and the belt tension of the first belt 71 and the second belt 72 is not able to provide a sufficient nip pressure at the free nip portion.
Patent Document 3 (Japanese Patent Application Publication 2007-219109), Patent Document 4 (Japanese Patent Application Publication 2007-34276), and Patent Document 5 (Japanese Patent Application Publication 2007-240622) each disclose a fixing device having a structure in which the inner surface of the first belt is pressed using a pressure pad so as to press the outer circumferential surface of the first belt in against the outer circumferential surface of the second belt.
As each of Patent Documents 3-5 discloses, pressing the inner circumferential surface of the first belt using a pressure pad allows the respective portions of the first belt and the second belt that oppose each other to be mutually in pressing contact, which increases the nip pressure. However, because the pressure pad is slidably in contact with the inner circumferential surface of the first belt, the pressure pad and the first belt may wear with passage of time. As a result, a desired nip pressure may not be stably achieved for a long period of time. Additionally, pressing the inner circumferential surface of the first belt using the pressure pad increases the heat capacity required to heat the first belt because of the heat capacity of the pressure pad. As a result, a heating time (warm-up time) required to raise the temperature of the fixing nip portion to a predetermined temperature becomes longer as well.